The invention relates to a method for stimulating the production of an underground zone containing petroleum effluents and adjoining an aquifer, by lateral sweeping by means of a fluid capable of displacing the effluents, such as a warm fluid, a solvent, etc.
The method according to the invention particularly applies to the stimulation of the production, through a deflected drain, of a zone where displacing dense petroleum effluents is obtained by injecting into the formation a displacement fluid by means of another deflected drain.
What is called a deflected drain is any drain whose part running across the producing formation is horizontal or at least very inclined in relation to the vertical.
It is well-known that, in reservoirs where a horizontal boundary or interface initially exists between a producing zone and an aquifer under pressure, the interface undergoes a deformation at the time of a withdrawal through a well close to the interface (coning or edge effect). If the dynamic depression resulting from the flow is sufficient, water may flow into the producing drain and mix with the sought effluent. This leads to moving the production drain away from the interface as much as possible to avoid a water inflow.
A method for decreasing the intake, in a drain conveying a petroleum effluent, of another, undesirable fluid coming from an adjoining zone under the effect of the pressure gradient caused by the withdrawal is well-known through French patent FR 2,555,247. This is achieved by piercing through the production zone a first deflected drain and at least a second drain closer to the interface with the adjoining zone than the first drain. Part of the second drain may be on the interface or even in the adjoining zone. Both drains are then made to produce. The undesirable fluid is directly collected through the second drain when it is drilled in the adjoining zone. When the second drain is drilled in the production zone itself or near the interface, the undesirable fluid intake generated by the withdrawal performed in the first drain is mainly collected through the second drain. In all cases, the interface between the two fluids is stabilized. The inflow of undesirable fluid in the first drain is suppressed or highly decreased.
One technique used for assisting the production of petroleum effluents that are little mobile or much less mobile than the water which may be present in an adjoining zone essentially consists in injecting steam into the formation to increase the mobility of the effluents immobilized in the pores. Various methods utilizing this technique are described for example in U.S. Pat. Nos. 4,733,724; 4,718,489; 4,641,709; 4,607,699; 4,574,884; 4,344,485, etc.
Sweeping through the displacement fluid can for example be achieved between two drains laterally offset in relation to one another and substantially parallel. U. S. Pat. No. 4,574,884 for example describes a method comprising drilling two horizontal drains substantially in the same horizontal plane and laterally offset in relation to one another and substantially parallel. After establishing fluid pressure transmission or communication channels between the two drains, a fluid displacement is generated (by combustion) in the formation around the first drain in order to sweep the formation and the displaced effluents are collected through the other drain.
The lateral sweeping of a production zone by the displacement fluid is often difficult and little profitable because of the presence of a subjacent aquifer. It may sometimes happen that the displacement fluid quickly ends up in the aquifer fruitlessly. It may also happen that it displaces the oil from the formation towards the aquifer. This oil is then trapped in contact with water. Besides, withdrawing the swept oil through the production drain has the effect, as seen above, of deforming the interface through a coning effect and of driving the water out of the aquifer.